The present invention relates to a method for manufacturing a plastic fuel tank with improved creep strength.
Plastic fuel tanks intended for motor vehicles have to meet specifications that specify maximum permissible amplitudes of deflection on their lower skin. The deflections stated in these specifications usually have to be met during ageing tests in which the tank contains a certain quantity of fuel for a given period of time (typically several weeks) and at temperature (usually 40° C.). The purpose of these specifications is to ensure that vehicles maintain their road clearance and to prevent the skin of the tank coming into contact with hotspots of the vehicle.
At the present time, plastic fuel tanks are generally fixed to the chassis of the vehicle via plastic lugs and are supported by metal straps. The latter are used in particular on the more capacious tanks where compliance with maximum permissible deflections is more difficult. However, recourse to these straps involves an additional attachment step and is therefore not very economical.
Solutions have been proposed in the prior art with a view to enhancing the mechanical strength (including the creep strength) of fuel tanks.
Thus, it has been proposed (particularly in U.S. Pat. Nos. 3,919,373 and 4,891,000) that an insert be secured inside the tank at the time of its blow-moulding from a cylindrical parison. However, it is not easy to position the insert accurately using this technique.
It has also been proposed (in US Application 2002/0100759) that the two walls of the tank be secured together directly by welding. However, this results in a loss of useful tank volume (as a result of the inflection of the said walls at the welding seam).